Abstract
In many land plants, the stomatal pore opens during the day and closes during the night. Thus, periods of darkness could be effective in decreasing pathogen penetration into leaves through stomata, the primary sites for infection by many pathogens. Pseudomonas syringae pv. tomato (Pst) DC3000 produces coronatine (COR) and opens stomata, raising an intriguing question as to whether this is a virulence strategy to facilitate bacterial infection at night. In fact, we found that (a) biological concentration of COR is effective in opening dark-closed stomata of Arabidopsis thaliana leaves, (b) the COR defective mutant Pst DC3118 is less effective in infecting Arabidopsis in the dark than under light and this difference in infection is reduced with the wild type bacterium Pst DC3000, and (c) cma, a COR biosynthesis gene, is induced only when the bacterium is in contact with the leaf surface independent of the light conditions. These findings suggest that Pst DC3000 activates virulence factors at the pre-invasive phase of its life cycle to infect plants even when environmental conditions (such as darkness) favor stomatal immunity. This functional attribute of COR may provide epidemiological advantages for COR-producing bacteria on the leaf surface.
Highlights
Plants are exposed to different types and combinations of environmental conditions every day
We have previously shown that stomatal immunity leads to reduction of P. syringae infection of the leaf apoplast and COR can override this pathogen-associated molecular patterns (PAMP)-triggered stomatal closure leading to bacterial penetration into the leaf tissue (Melotto et al, 2006)
We found that Pst DB29 behaves to Pst DC3118 in inducing a lasting stomatal closure (Figure 1A) as well as not being effective in colonizing the apoplast of surface-inoculated Arabidopsis plants (Figure 1B)
Summary
Plants are exposed to different types and combinations of environmental conditions every day. Plants rely on specific mechanisms that sense these changes in environmental conditions and relay the message to plant cells, leading to adaptation to those conditions for optimum growth and reproduction. This is extremely crucial in case of crop plants, where yields are largely dependent on environmental conditions. Along with abiotic factors affecting plant productivity, plants are continuously and simultaneously exposed to biotic stresses. Cold and heat stress leads to lower resistance of plant to viruses (Szittya et al, 2003). It has been shown that plant responses to such combinations of stimuli are more complicated and different than the response to a single
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